1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition for use in a piezoelectric sensor, a piezoelectric buzzer, a piezoelectric actuator, a ceramic oscillator, a ceramic filter, or the like, and particularly to a piezoelectric ceramic composition adapted to a piezoelectric element for use in a vibration detecting piezoelectric sensor.
2. Description of the Related Art
A piezoelectric ceramic composition for use to constitute a piezoelectric sensor, especially, to constitute a vibration detecting piezoelectric sensor is required to have a small mechanical quality coefficient (Qm) (for example, not larger than 200) as its characteristic in order to detect objective vibration stably without being affected by noise.
In order to obtain high sensitivity of the sensor, it is preferable to select the relative dielectric constant E of the piezoelectric ceramic composition to be as large as possible (for example, not smaller than 1200) and the electromechanical coupling factor k31 of stretching vibration in a long side direction in a piezoelectric vibration mode to be high (for example, not lower than 35%).
Further, in the piezoelectric ceramic composition for use in a vibration detecting sensor, in order to enhance sensor sensitivity, a piezoelectric ceramic composition element is required to have large strength (for example, not smaller than 10 kgf/mm.sup.2) because the element is required to be thin (for example, 60 to 100 .mu.m).
On the other hand, high-density mounting of electronic parts onto a board has been made progress with the advance in the reduction in size and thickness of electronic appliances in recent years. Also as vibration detecting sensors, surface mounting type sensors are used mainly. Surface mounting type electronic parts must be excellent in heat resistance because the parts are exposed to a temperature of about 250.degree. C. in a reflow furnace when the parts are mounted onto a board. That is, in order to obtain stable sensor sensitivity, it is preferable that the deterioration of piezoelectric characteristic after surface mounting is little.
It is heretofore known that a lead zirconate titanate type composition is suitable as a piezoelectric ceramic composition for use in a ceramic oscillator or a ceramic filter. Various improvements have been proposed in accordance with the purpose of use thereof.
For example, there have been proposed lead zirconate titanate type compositions which are hardened (sintered) by addition of MnO.sub.2, Cr.sub.2 O.sub.3, or the like, to the compositions; lead zirconate titanate type compositions which are improved in piezoelectric characteristic by solid solution of complex perowskite type compounds such as Pb(Ni.sub.1/2 Nb.sub.2/3)O.sub.3, Pb(Mg.sub.1/2 Nb.sub.2/3)O.sub.3, or the like, as a third component in the compositions and further hardened by addition of MnO.sub.2, Cr.sub.2 O.sub.3, or the like, to the compositions; and compositions obtained by replacing a part of Pb by Ba, Sr or Ca in order to improve the temperature characteristic of resonant frequency fr in the above-mentioned compositions.
Further, Unexamined Japanese Patent Publication No. Hei. 10-120463 discloses a piezoelectric ceramic composition small in mechanical quality coefficient and excellent in heat resistance.
A conventionally known piezoelectric ceramic composition low in mechanical quality coefficient Qm has a problem that the Curie temperature Tc is low (Tc&lt;200.degree. C.) because the composition only aims at increasing both the relative dielectric constant and the electromechanical coupling factor as represented by piezoelectric ceramics for use in an actuator.
Further, a composition having a Curie temperature Tc of about 300.degree. C. and a small mechanical quality coefficient Qm has a problem that the heat resistance of the element is poor and the change with time of resonant frequency fr (the change of resonant frequency with respect to time) is large.
Some piezoelectric ceramic compositions are high in Curie temperature Tc and relatively good in heat resistance among the conventional piezoelectric ceramic compositions which are hardened by addition of MnO.sub.2, Cr.sub.2 O.sub.3, or the like, to PbTiO.sub.3 --PbZrO.sub.3 type materials and the conventional piezoelectric ceramic compositions which are improved in piezoelectric characteristic by solid solution of complex perowskite type compounds such as Pb(Ni.sub.1/2 Nb.sub.2/3)O.sub.3, Pb(Mg.sub.1/2 Nb.sub.2/3)O.sub.3, or the like, as a third component in PbTiO.sub.3 --PbZrO.sub.3 type materials and further hardened by addition of MnO.sub.2, Cr.sub.2 O.sub.3, or the like, to the materials. In those cases, however, there is a problem that the mechanical quality coefficient Qm is large (for example, Qm&gt;200). It was difficult to configure a product as a target of the present invention.
Further, the piezoelectric ceramic composition disclosed in Unexamined Japanese Patent Publication No. Hei. 10-120463 is a piezoelectric ceramic composition which is small in mechanical quality coefficient Qm and excellent in heat resistance. The composition is, however, inferior in element strength, and it is difficult to produce an element which has a thickness not larger than 100 .mu.m and which can be applied to a product as a target of the present invention.